Representative samples of tumor tissue and appropriate normal tissues are being collected at the terminal sacrifices of National Toxicology Program rat and mouse carcinogenesis bioassays and frozen for subsequent molecular biology studies. Studies to identify activation of oncogenes and loss or alteration of tumor suppressor genes in these samples are underway. Samples are being collected for use by various NIEHS investigators. Special efforts will be made to obtain chemically induced tumors when it is apparent that a carcinogenic effect is being manifested. Work on these samples as well as similar samples collected from in-house and other contractor-generated tissues has been underway for the past 5 years and has yielded information helpful for interpretation of bioassay results and useful in risk assessment. It is anticipated that collection efforts will continue over the next 4 years.